This invention relates to a radio communication system comprising a fixed unit connected to an exchange through a subscriber line and a portable unit, such as a cordless telephone set, communicable with the fixed unit through a plurality of radio channels.
In a conventional radio communication system of the type described, a small number of radio channels, for example, eight radio channels are assigned to communication between a portable unit and a fixed unit. In addition, the portable unit is driven by a battery and is repeatedly and intermittently put into an active state and an inactive one so as to save power consumption of the battery. Such an intermittent operation will be called a battery saving operation. The battery saving operation is interrupted either when the portable unit goes off-hook or when a call is received from the fixed unit. Anyway, an idle one of the radio channels must be searched and seized prior to the communication. Therefore, a radio signal is transmitted between the fixed and the portable units through a seized one of the radio channels.
Seizure of the idle radio channel can comparatively rapidly be carried out when a small number of the radio channels are used between the fixed and the portable units. However, unfavorable channel interference often takes place when the portable unit is close to another portable unit.
It is a recent trend that radio channels have increased in number to forty or so. However, an increase of the radio channels brings about lengthening a time of seizing an idle one of the radio channels.
The time of seizure might be reduced if scanning operation of the radio channels is carried out independently of each of the fixed and the portable units. According to the inventor's experimental studies, it takes as long as a few seconds to seize the idle channel even on carrying out the scanning operation. This is because the scanning operation is started after occurrence of a telephone call.